This invention relates to metal acrylates which are useful as processing aids for elastomers.
Elastomers such as polybutadiene, hydrogenated nitrile rubber, etc., are usually processed in a two roll mill or a Banbury type internal mixer, with a combination of additives which are added in order to provide the proper curing and physical property characteristics. For peroxide cure systems, metal salts, especially acrylates, are commonly used to improve the efficiency of the cure process. Additional ingredients added to the compound include peroxides, fillers such as zinc oxide, plasticizers, additional coagents, and pigments.
In typical processing of such elastomers, there is a tendency for build-up to occur on the rolls, sides of the mixer, and/or on the rotors. This build-up, if left over several cycles, reduces the heat transfer of the equipment and undesirably lengthens the cycle time. Also, fragments from the build-up can enter subsequent batches and result in non-uniform properties and out-of-specification product. In such cases, it may become necessary to shut down and either run clean-out batches until the equipment is clean, or to mechanically clean the mixer.
Metal acrylates and their interaction with the peroxide and elastomer are frequently a major contributor to the undesirable build-up (also known as plate-out process).
Various methods have been tried in an effort to minimize the tendency for the metal acrylates to plate-out. One such method involves the ordering of the addition of the formulation ingredients. Normally, most of the ingredients are added together to the elastomer with the peroxide being added last. Under these conditions, plate-out is very bad. By adding the metal acrylate first, it is possible to partially encapsulate and minimize the contact of metal acrylate with the ingredients causing the plate-out. This technique has the disadvantage of lengthening the mixing cycle and only partially reducing plate-out.
An alternate technique is surface treating of the metal acrylate particle surface prior to addition to the mixer. Leo, et al., U.S. Pat. No. 4,092,285 assigned to Wyrough and Loser, Inc., discloses the encapsulation of "critical chemical" rubber additives with a mixture of a liquid compatible with the rubber or a wax with a melting point of 55-80.degree. C., and a high molecular weight polymer with molecular weight greater than 50,000. This mixture produces a non-tacky gel below 53.degree. C. which can be coated onto the particle surface. Johansson, et al., U.S. Pat. No. 4,397,992, also assigned to Wyrough and Loser, Inc., discloses the incorporation of a vulcanizing agent or coagent into a polymeric binder composition comprising a major amount of multi-stage graft copolymer along with a high molecular weight hydrocarbon polymer and free radical polymerization inhibitor. This composition results in a non-bleeding homogeneous solid which can be readily dispersed in a rubber or plastic stock by mechanically mixing. However, in the case of metal acrylates, neither the Leo, et al., nor Johansson, et al., processes provide surface treatment of encapsulated metal acrylates which function effectively to prevent plate-out in elastomer processing. Neither of these patents suggests a method which provides an efficient method of coating or treating the metal acrylate particles prior to their incorporation into elastomers.
Metal acrylates, as normally produced, are typically dusty products (approximately 10 microns in diameter) causing odor and inhalation problems when transferred because of dust. In addition, metal acrylates are usually extremely difficult to process on mixing equipment, such that they tend to stick to the mixer metal surfaces (plate-out) resulting in poor dispersion of the metal acrylates in the rubber or elastomer. An additional disadvantage of prior art metal acrylates is the slowness with which they mix into many rubbers due to the polar nature of the particles and charge build-up on the particle surface.